Battery overcharge protection. Protection against deep battery discharge. How to choose a suitable field-effect transistor
There are two things that batteries really don't like: overcharging and overdischarging. And if the first problem is successfully solved by modern chargers(except for the simplest rectifiers), then with a discharge below the critical level things are worse - almost never battery-powered devices provide protection against overdischarge. An accidental discharge cannot be ruled out - when you simply forgot to turn off the device and it discharges, discharges... To solve this problem, it is proposed to self-assembly simple low voltage circuit breaker module. This circuit is quite simple and can be applied to any lithium or lead-acid battery. Naturally, the shutdown threshold can be adjusted according to the battery.
Battery protection unit diagram
How does this work. When the reset button is pressed, positive voltage is applied to the gate of the N-channel MOSFET power transistor.
If the voltage at the output of Zener diode U1 is higher than 2.5 volts, as determined by the voltage divider consisting of R4, R5 and R6, the cathode of U1 is connected to its anode, making it negative with respect to its emitter, R2 limits the base current to a safe value and provides sufficient current to operate U1. And transistor Q1 will keep the circuit open even when you release the reset button.
If the voltage at U1 drops below 2.5 volts, the zener diode turns off and pulls up the positive voltage at the emitter of R1, turning it off. Resistor R8 also turns off field effect transistor, leading to load shedding. Moreover, the load will not be turned on again until the reset button is pressed.
Most small FETs are rated for only +/- 20 volts at the gate source voltage, meaning the block circuit is suitable for no more than 12 volt devices: if higher operating voltages are required, additional circuit elements will need to be added to maintain safety fieldworker's work. An example of using such a circuit: a simple charge controller solar panels shown in the photo.
If a lower voltage than 9 volts (or higher than 15) is required, it will be necessary to recalculate the values of resistors R4 and R6 to change the adjustment range.
You can put almost any silicon PNP transistor with a rating of at least 30 volts and any N-channel MOSFET with a rated voltage of at least 30 volts and a current more than 3 times that which you are going to switch into the circuit. Feedthrough resistance of a fraction of Ohm. For the prototype, the F15N05 was used - 15 amps, 50 volts. For high currents, transistors IRFZ44 (50 A Max.) and PSMN2R7-30PL (100 A Max.) are suitable. You can also connect several similar field-effect transistors in parallel as needed.
This device should not remain connected to the battery for a long time, since it itself consumes several milliamps due to the LED and the current consumption of U1. When turned off, its current consumption is negligible.
Everyone knows that deep discharge of batteries sharply reduces their service life. In order to exclude this mode of operation of batteries, use various schemes– discharge limiters. With the advent of microcircuits and powerful field-effect switching transistors, such circuits began to have small dimensions and became more economical.
The limiter circuit, which has already become a classic, is shown in Figure 1; it can be found in many amateur radio circuits. The device is intended to operate as part of uninterruptible source feeding a home incubator. Field effect transistor VT1 - IRF4905 in this circuit performs the function of a switch, and the KR142EN19 microcircuit is a voltage comparator.
When contacts K1 are closed, these are relay contacts that connect the battery in the absence of 220V mains voltage, the circuit is supplied with voltage from the battery GB1, but since the transistor switch itself cannot open, two are introduced to start it additional element– C1 and R2. And so, when voltage appears at the input, capacitor C1 begins to charge. At the first moment of its charging, the gate of the transistor is shunted by this capacitor to the common wire of the circuit. The transistor opens and if the voltage on the battery is above the threshold set on the comparator, it remains open further, but if the voltage is lower... then the transistor immediately closes. The threshold for disconnecting the battery from the load is set by resistor R3. The comparator works as follows. As the battery discharges, the voltage at pin 1 of the DA1 KR142EN19 microcircuit will decrease and as soon as it approaches the reference voltage of this chip -2.5V, the voltage at its pin 3 will begin to increase, which corresponds to a decrease in the voltage in the source-gate section of transistor VT1. The transistor will begin to close, which will lead to an even greater decrease in the voltage at pin 1 of DA1. An avalanche-like process of closing VT1 occurs. As a result, the load will be disconnected from the battery. The load current switched by this transistor can be increased several times, provided that thermal regime transistor. I mean installing it on a radiator, but do not forget that at a crystal temperature of 100°C, the maximum drain current decreases to 52A. The transistor drain power of 200W is given in the reference book for a temperature of 25°C.
Resistor R1 is needed to create the required current through the microcircuit, which must be at least one milliamp. Capacitors C1 and C3 are blocking. R4 is the load resistance. If you connect a diode in series with the load, preferably with a Schottky barrier, then you can enter into this circuit an indicator for the transition of work to battery– LED HL1. To save battery energy, it is better to use a super-bright LED as an indicator and select the value of the resistor R according to the desired brightness.
Drawing printed circuit board You can download the battery discharge limiter here.
I needed to protect my battery from deep discharge. And the main requirement for the protection circuit is that after the battery is discharged, it turns off the load and cannot turn it on on its own after the battery has built up a little voltage at the terminals, without a load.
The circuit is based on the 555th timer, connected as a single pulse generator, which, after reaching the minimum threshold voltage, will close the gate of transistor VT1 and turn off the load. The circuit will be able to turn on the load only after disconnecting and reconnecting the power.
Fee (No need to mirror): 
SMD Board (Need mirroring): 
All SMD resistors are 0805. The MOSFET package is D2PAK, but DPAK is also possible.
When assembling, you should pay attention to the fact that there is a jumper under the chip (in the board with DIP components) and the main thing is not to forget about it!
The circuit is configured as follows: resistor R5 is set to the top position according to the circuit, then we connect it to a power source with a voltage set on it, at which it should turn off the load. If you believe Wikipedia, then the voltage of a completely discharged 12-Volt battery corresponds to 10.5 Volts, this will be our load disconnect voltage. Next, rotate the R5 regulator until the load is turned off. Instead of the IRFZ44 transistor, you can use almost any powerful low-voltage MOSFET, you just need to take into account that it must be designed for a current 2 times greater than the maximum load current, and the gate voltage must be within the supply voltage.
If desired, the trimming resistor can be replaced with a constant one with a nominal value of 240 kOhm, and in this case resistor R4 must be replaced with 680 kOhm. Provided that the threshold of the TL431 is 2.5 Volts.
The current consumption of the board is about 6-7 mA.
Protection of lithium-ion batteries (Li-ion). I think that many of you know that, for example, inside the battery from mobile phone There is also a protection circuit (protection controller), which ensures that the battery (cell, bank, etc...) is not overcharged above a voltage of 4.2 V, or discharged below 2...3 V. Also, the protection circuit saves from short circuits by turning off the jar itself from the consumer at the time of the short circuit. When the battery reaches the end of its service life, you can remove the protection controller board from it and throw away the battery itself. The protection board can be useful for repairing another battery, for protecting a can (which does not have protection circuits), or you can simply connect the board to the power supply and experiment with it.
I had many protection boards for batteries that had become unusable. But a search on the Internet for the markings of the microcircuits yielded nothing, as if the microcircuits were classified. On the Internet there was documentation only for assemblies of field-effect transistors, which are included in the protection boards. Let's look at the design of a typical lithium-ion battery protection circuit. Below is a protection controller board assembled on a controller chip designated VC87 and a transistor assembly 8814 ():
In the photo we see: 1 - protection controller (the heart of the entire circuit), 2 - assembly of two field-effect transistors (I will write about them below), 3 - resistor setting the protection operation current (for example during a short circuit), 4 - power supply capacitor, 5 - resistor (for powering the controller chip), 6 - thermistor (found on some boards to control the battery temperature).
Here is another version of the controller (there is no thermistor on this board), it is assembled on a chip with the designation G2JH, and on a transistor assembly 8205A ():
Two field-effect transistors are needed so that you can separately control the charging protection (Charge) and the discharge protection (Discharge) of the battery. There were almost always datasheets for transistors, but none for controller chips!! And the other day I suddenly came across an interesting datasheet for some kind of lithium-ion battery protection controller ().
And then, out of nowhere, a miracle appeared - after comparing the circuit from the datasheet with my protection boards, I realized: The circuits match, they are one and the same thing, clone chips! After reading the datasheet, you can use similar controllers in your homemade products, and by changing the value of the resistor, you can increase the permissible current that the controller can deliver before the protection is triggered.
It's no secret that Li-ion batteries do not like deep discharge. This causes them to wither and wither, and also increase internal resistance and lose capacity. Some specimens (those with protection) can even plunge into deep hibernation, from where it is quite problematic to pull them out. Therefore, when using lithium batteries, it is necessary to somehow limit their maximum discharge.
To do this, special circuits are used that disconnect the battery from the load at the right time. Sometimes such circuits are called discharge controllers.
Because The discharge controller does not control the magnitude of the discharge current; strictly speaking, it is not a controller of any kind. In fact, this is an established but incorrect name for deep discharge protection circuits.
Contrary to popular belief, the built-in batteries (PCB boards or PCM modules) are not designed to limit the charge/discharge current, or to timely turn off the load when fully discharged, or to correctly determine the moment of the end of the charge.
Firstly, Protection boards, in principle, are not capable of limiting the charge or discharge current. This should be handled by the memory department. The maximum they can do is turn off the battery when short circuit under load or when it overheats.
Secondly, Most protection modules turn off the li-ion battery at a voltage of 2.5 Volts or even less. And for the vast majority of batteries, this is a very strong discharge; this should not be allowed at all.
Thirdly, The Chinese are riveting these modules in the millions... Do you really believe that they use high-quality precision components? Or that someone out there tests and adjusts them before installing them in batteries? Of course, this is not true. When producing Chinese motherboards, only one principle is strictly observed: the cheaper, the better. Therefore, if the protection disconnects the battery from the charger exactly at 4.2 ± 0.05 V, then this is more likely a happy accident than a pattern.
It’s good if you got a PCB module that will operate a little earlier (for example, at 4.1V). Then the battery simply won’t reach ten percent of its capacity and that’s it. It is much worse if the battery is constantly recharged, for example, to 4.3V. Then the service life is reduced and the capacity drops and, in general, may swell.
It is IMPOSSIBLE to use the protection boards built into lithium-ion batteries as discharge limiters! And as charge limiters too. These boards are intended only for emergency battery disconnection in case of emergency situations.
Therefore we need separate circuits charge limitations and/or protection against too deep discharge.
We looked at simple chargers based on discrete components and specialized integrated circuits in. And today we’ll talk about the solutions that exist today to protect a lithium battery from too much discharge.
To begin with, I propose a simple and reliable Li-ion overdischarge protection circuit, consisting of only 6 elements. 
The values indicated in the diagram will result in the batteries being disconnected from the load when the voltage drops to ~10 Volts (I made protection for 3 series-connected 18650 batteries in my metal detector). You can set your own shutdown threshold by selecting resistor R3.
By the way, the tension is full Li-ion discharge The battery is 3.0 V and no less.
A field grass (such as in the diagram or similar) can be dug out from an old motherboard from the computer, there are usually several of them at once. TL-ku, by the way, can also be taken from there.
Capacitor C1 is needed for the initial startup of the circuit when the switch is turned on (it briefly pulls the gate T1 to minus, which opens the transistor and powers the voltage divider R3, R2). Further, after charging C1, the voltage required to unlock the transistor is maintained by the TL431 microcircuit.
Attention! The IRF4905 transistor indicated in the diagram will perfectly protect three lithium-ion batteries connected in series, but is completely unsuitable for protecting one 3.7 Volt bank. It is said how to determine for yourself whether a field-effect transistor is suitable or not.
The disadvantage of this circuit: in the event of a short circuit in the load (or too much current consumption), the field-effect transistor will not close immediately. The reaction time will depend on the capacitance of capacitor C1. And it is quite possible that during this time something will have time to burn out properly. A circuit that instantly responds to a short load under load is presented below: 
Switch SA1 is needed to “restart” the circuit after the protection has tripped. If the design of your device provides for removing the battery to charge it (in a separate charger), then this switch is not needed.
The resistance of resistor R1 must be such that the TL431 stabilizer reaches operating mode at a minimum battery voltage - it is selected in such a way that the anode-cathode current is at least 0.4 mA. This gives rise to another drawback of this circuit - after the protection is triggered, the circuit continues to consume energy from the battery. The current, although small, is quite enough to completely drain a small battery in just a couple of months.
The diagram below for homemade control of the discharge of lithium batteries is free from this drawback. When the protection is triggered, the current consumed by the device is so small that my tester does not even detect it. 
Below are more modern version discharge limiter lithium battery using stabilizer TL431. This, firstly, allows you to easily and simply set the desired response threshold, and secondly, the circuit has high temperature stability and clear shutdown. Clap and that's it! 
Getting TL-ku today is not a problem at all, they are sold for 5 kopecks per bunch. Resistor R1 does not need to be installed (in some cases it is even harmful). Trimmer R6, which sets the response voltage, can be replaced with a chain of constant resistors with selected resistances.
To exit the blocking mode, you need to charge the battery above the protection threshold, and then press the S1 “Reset” button.
The inconvenience of all the above schemes is that to resume operation of the schemes after going into protection, operator intervention is required (turn SA1 on and off or press a button). This is the price to pay for simplicity and low power consumption in lock mode.
The simplest li-ion overdischarge protection circuit, devoid of all shortcomings (well, almost all) is shown below: 
The principle of operation of this circuit is very similar to the first two (at the very beginning of the article), but there is no TL431 microcircuit, and therefore its own current consumption can be reduced to very small values - about ten microamps. A switch or reset button is also not needed; the circuit will automatically connect the battery to the load as soon as the voltage across it exceeds a preset threshold value.
Capacitor C1 suppresses false alarms when operating on a pulsed load. Any low-power diodes will do; it is their characteristics and quantity that determine the operating voltage of the circuit (you will have to select it locally).
Any suitable n-channel field effect transistor can be used. The main thing is that it can withstand the load current without straining and be able to open at low gate-source voltage. For example, P60N03LDG, IRLML6401 or similar (see).
The above circuit is good for everyone, but there is one unpleasant moment - the smooth closing of the field-effect transistor. This occurs due to the flatness of the initial section of the current-voltage characteristic of the diodes.
This drawback can be eliminated with the help of modern element base, namely with the help of micro-power voltage detectors (power monitors with extremely low power consumption). The next scheme for protecting lithium from deep discharge is presented below: 
MCP100 microcircuits are available in both DIP packages and planar versions. For our needs, a 3-volt option is suitable - MCP100T-300i/TT. Typical current consumption in blocking mode is 45 µA. The cost for small wholesale is about 16 rubles/piece.
It’s even better to use a BD4730 monitor instead of the MCP100, because it has a direct output and, therefore, it will be necessary to exclude transistor Q1 from the circuit (connect the output of the microcircuit directly to the gate of Q2 and resistor R2, while increasing R2 to 47 kOhm).
The circuit uses a micro-ohm p-channel MOSFET IRF7210, which easily switches currents of 10-12 A. The field switch is fully open already at a gate voltage of about 1.5 V, and in the open state it has negligible resistance (less than 0.01 Ohm)! In short, a very cool transistor. And, most importantly, not too expensive.
In my opinion, the last scheme is the closest to the ideal. If I had unlimited access to radio components, I would choose this one.
A small change in the circuit allows you to use an N-channel transistor (then it is connected to the negative load circuit): 
BD47xx power supply monitors (supervisors, detectors) are a whole line of microcircuits with response voltages from 1.9 to 4.6 V in steps of 100 mV, so you can always choose them to suit your purposes.
A small retreat
Any of the above circuits can be connected to a battery of several batteries (after some adjustment, of course). However, if the banks have different capacities, then the weakest of the batteries will constantly go into a deep discharge long before the circuit operates. Therefore, in such cases, it is always recommended to use batteries not only of the same capacity, but preferably from the same batch.
And although such protection has been working flawlessly in my metal detector for two years now, it would still be much more correct to monitor the voltage on each battery personally.
Always use your personal discharge controller Li-ion battery for each jar. Then any of your batteries will serve you happily ever after.
How to choose a suitable field-effect transistor
In all of the above schemes for protecting lithium-ion batteries from deep discharge, MOSFETs operating in switching mode are used. The same transistors are usually used in overcharge protection circuits, short-circuit protection circuits, and in other cases where load control is required.
Of course, in order for the circuit to work as it should, the field-effect transistor must meet certain requirements. First, we will decide on these requirements, and then we will take a couple of transistors and, according to their datasheets (according to technical specifications) let's determine whether they are suitable for us or not.
Attention! We will not consider the dynamic characteristics of FETs, such as switching speed, gate capacitance and maximum pulsed drain current. These parameters become critically important when the transistor operates at high frequencies (inverters, generators, PWM modulators, etc.), however, discussion of this topic is beyond the scope of this article.
So, we must immediately decide on the circuit that we want to assemble. Hence the first requirement for a field-effect transistor - it must be the right type(either N- or P-channel). This is the first one.
Let's assume that the maximum current (load current or charge current - it doesn't matter) will not exceed 3A. This leads to the second requirement - the field worker must long time withstand such current.
Third. Let's say our circuit will protect the 18650 battery from deep discharge (one bank). Therefore, we can immediately decide on the operating voltages: from 3.0 to 4.3 Volts. Means, maximum permissible drain-source voltage U ds should be more than 4.3 Volts.
However, the last statement is true only if only one lithium battery bank is used (or several connected in parallel). If, to power your load, a battery of several batteries connected in series is used, then maximum voltage The drain-source of the transistor must exceed total voltage entire battery.
Here is a picture explaining this point: 
As can be seen from the diagram, for a battery of 3 18650 batteries connected in series, in the protection circuits of each bank it is necessary to use field devices with a drain-to-source voltage U ds > 12.6V (in practice, you need to take it with some margin, for example, 10%).
At the same time, this means that the field-effect transistor must be able to open completely (or at least strongly enough) already at a gate-source voltage U gs of less than 3 Volts. In fact, it is better to focus on a lower voltage, for example, 2.5 Volts, so that there is a margin.
For a rough (initial) estimate, you can look in the datasheet at the “Cut-off voltage” indicator ( Gate Threshold Voltage) is the voltage at which the transistor is on the threshold of opening. This voltage is typically measured when the drain current reaches 250 µA.
It is clear that the transistor cannot be operated in this mode, because its output impedance is still too high, and it will simply burn out due to excess power. That's why The transistor cut-off voltage must be less than the operating voltage of the protection circuit. And the smaller it is, the better.
In practice, to protect one can of a lithium-ion battery, you should select a field-effect transistor with a cutoff voltage of no more than 1.5 - 2 Volts.
Thus, the main requirements for field-effect transistors are as follows:
- transistor type (p- or n-channel);
- maximum permissible drain current;
- the maximum permissible drain-source voltage U ds (remember how our batteries will be connected - in series or in parallel);
- low output resistance at a certain gate-source voltage U gs (to protect one Li-ion can, you should focus on 2.5 Volts);
- maximum permissible power dissipation.
Now let's get on specific examples. For example, we have at our disposal the transistors IRF4905, IRL2505 and IRLMS2002. Let's take a closer look at them.
Example 1 - IRF4905
We open the datasheet and see that this is a transistor with a p-type channel (p-channel). If we are satisfied with this, we look further.
The maximum drain current is 74A. In excess, of course, but it fits.
Drain-source voltage - 55V. According to the conditions of the problem, we have only one bank of lithium, so the voltage is even greater than required.
Next, we are interested in the question of what the drain-source resistance will be when the opening voltage at the gate is 2.5V. We look at the datasheet and don’t immediately see this information. But we see that the cutoff voltage U gs(th) lies in the range of 2...4 Volts. We are categorically not happy with this.
The last requirement is not met, so discard the transistor.
Example 2 - IRL2505
Here is his datasheet. We look and immediately see that this is a very powerful N-channel field device. Drain current - 104A, drain-source voltage - 55V. So far everything is fine.
Check the voltage V gs(th) - maximum 2.0 V. Excellent!
But let's see what resistance the transistor will have at a gate-source voltage = 2.5 volts. Let's look at the chart: 
It turns out that with a gate voltage of 2.5V and a current through the transistor of 3A, a voltage of 3V will drop across it. In accordance with Ohm's law, its resistance at this moment will be 3V/3A=1Ohm.
Thus, when the voltage on the battery bank is about 3 Volts, it simply cannot supply 3A to the load, since for this the total load resistance, together with the drain-source resistance of the transistor, must be 1 Ohm. And we only have one transistor that already has a resistance of 1 ohm.
In addition, with such an internal resistance and a given current, the transistor will release power (3 A) 2 * 3 Ohm = 9 W. Therefore, you will need to install a radiator (a TO-220 case without a radiator can dissipate somewhere around 0.5...1 W).
An additional alarm bell should be the fact that the minimum gate voltage for which the manufacturer specified the output resistance of the transistor is 4V.
This seems to hint that the operation of the field worker at a voltage U gs less than 4 V was not envisaged.
Considering all of the above, discard the transistor.
Example 3 - IRLMS2002
So, let's take our third candidate out of the box. And immediately look at its performance characteristics.
N-type channel, let's say everything is in order.
Maximum drain current - 6.5 A. Suitable.
The maximum permissible drain-source voltage V dss = 20V. Great.
Cut-off voltage - max. 1.2 Volts. So far so good.
To find out the output resistance of this transistor, we don’t even have to look at the graphs (as we did in the previous case) - the required resistance is immediately given in the table just for our gate voltage.
